(a) Field of the Invention
The present invention relates to a roughening-treated copper foil useful for printed wiring boards or the like and to the production thereof, particularly, to a roughening-treated copper foil, which is useful for printed wiring boards due to its excellent adhesive strength to resin base materials of high Tg""s (glass transition temperature), such as FR-5, and to a method suitable for the production thereof.
(b) Description of the Related Art
Copper foil for printed wiring boards generally has a bonding surface, which is previously roughened by some means to give higher adhesive strength on lamination onto resin base materials. For electrolytic copper foil, plating methods are mainly used as the roughening treatment. Japanese Patent Application Examined Publication No. 53-39376 (1978) discloses an example of the plating methods. According to the method, first a dendritic copper electrodeposition layer is formed on at least one bonding surface of a copper foil as a cathode by the so-called burning plating in an acidic copper plating bath at a current of the limiting current density or higher, then a smooth copper electrodeposition layer is formed on the dendritic copper electrodeposition layer at a current of a current density lower than the limiting current density (covering plating) to change the dendritic copper into nodular copper and to increase the adhesive strength by the nodular copper. After the formation of the nodular copper by the electrolysis treatment, the surface of the copper foil has a larger specific surface than before the electrolysis treatment, and the nodular copper works as an anchor, improving the adhesive strength between resin base materials and the copper foil. When nodular copper is formed on electrolytic copper foil, which generally has a surface (mat surface) rougher than the other surface (shiny surface), current is mainly centered to convexes, and the formation of the nodular copper is centered on the extreme ends of the convexes.
The recent spread of note type personal computers and pocket telephones has increased the use of glass-epoxy printed wiring boards produced by using as resin base materials FR-5 materials having high Tg""s. As compared with conventional FR-4 materials, epoxy resins having high Tg""s are more resistive to heat but have lower adhesive strength to copper foil. A means of enhancing the adhesive strength of copper foil to resin base materials is to increase the roughness of the bonding surface of copper foil. However, increasing the surface roughness tends to cause the so-called copper powder-falling off that is the falling of nodular copper even with small abrasion force and the so-called residual copper that is nodular copper left in resin base materials after the etching step in the production of printed circuits.
Japanese Patent Application Examined Publication No. 54-38053 (1979) discloses an improved method of forming a roughened surface by carrying out electrolysis treatment at an approximate limiting current density in an acidic copper plating bath to which a specific amount of at least one metal selected from arsenic, antimony, bismuth, selenium and tellurium is added. The addition of a very small amount of arsenic, antimony, bismuth, selenium or tellurium permits the formation of minute projections, but cannot solve the problem centering to the convexes on the copper foil. Further, using copper foil containing arsenic, antimony, bismuth, selenium or tellurium, which are poisons or deadly poisons, in printed wiring boards causes the problem of environmental pollution on discarding etching waste liquors or the printed wiring boards.
Addition of benzoquinoline to an acidic copper plating bath (Japanese Patent Application Examined Publication No. 56-41196 (1981)) or addition of molybdenum (Japanese Patent Application Examined Publication No. 62-56677 (1987)) are also proposed, but cannot improve adhesive strength sufficiently.
Japanese patent Application Unexamined Publication No. 8-236930 (1996) discloses a method for solving the problem, wherein electrolysis is carried out at approximate limiting current density in an acidic copper plating bath containing metal ions of at least one metal selected from chromium and tungsten and metal ions of at least one metal selected from vanadium, nickel, iron, cobalt, zinc, germanium and molybdenum, to form a roughening-treated layer containing the metals added. Japanese Patent Application Unexamined Publication No. 11-256389 (1999) discloses a method of carrying out electrolysis at approximate limiting current density in an acidic copper plating bath containing molybdenum ions and metal ions of at least one metal selected from iron, cobalt, nickel and tungsten, to form a layer of burnt deposits (a layer formed by burning plating) containing the metals added.
These methods, however, also cause the phenomena of copper powder-falling off and residual copper because nodular copper is formed exclusively on the extreme ends of the convexes on copper foil.
An object of the present invention is to solve the problems in the prior arts. That is, an object of the present invention is to provide a roughening-treated copper foil, which is suitable for printed wiring boards due to its high adhesive strength to resin base materials, by treating the bonding surface of copper foil by electrolysis so as to form nodular copper not only on the convexes on the bonding surface but also on the concaves thereon without increasing the surface roughness.
Another object of the present invention is to provide a method suitable for producing the roughening-treated copper foil.
Accordingly, the present invention provides a roughening-treated copper foil, comprising
(A) a copper foil,
(B) a composite metal layer, which is formed on a bonding surface of the copper foil and comprises (I) copper, (II) at least one metal selected from the group consisting of tungsten and molybdenum and (III) at least one metal selected from the group consisting of nickel, cobalt, iron and zinc, and
(C) a roughened layer comprising copper, which is formed on the composite metal layer.
Herein, the terms xe2x80x9cthe bonding surface of a copper foilxe2x80x9d mean a surface of a copper foil, which faces an adherent when the copper foil is bonded to the adherent.
The present invention further provides a method of producing a roughening-treated copper foil, comprising
treating a copper foil as a cathode by electrolysis in a plating bath containing (i) copper ions, (ii) metal ions of at least one metal selected from the group consisting of tungsten and molybdenum and (iii) metal ions of at least one metal selected from the group consisting of nickel, cobalt, iron and zinc, at a current density lower than a limiting current density of the plating bath, to form on the copper foil a composite metal layer comprising (I) copper, (II) at least one metal selected from the group consisting of tungsten and molybdenum and (III) at least one metal selected from the group consisting of nickel, cobalt, iron and zinc; and
forming a roughened layer comprising copper on the composite metal layer by carrying out electrolysis in a plating bath containing copper ions at a current density not lower than a limiting current density of the plating bath to form a dendritic copper electrodeposition layer, and then carrying out subsequent electrolysis at a current density lower than the limiting current density of the plating bath to form nodular copper. Herein, the terms xe2x80x9cthe limiting current density of the plating bathxe2x80x9d mean a current density at which generation of hydrogen accompanies the cathode reaction for the deposition of the metals or metal compounds.